As a processing target object having a thermal oxide film and a silicon nitride film (SiN) formed on the thermal oxide film, a semiconductor wafer is well-known. It may be required to etch and remove only the silicon nitride film from this semiconductor wafer without removing the thermal oxide film.
Known as an etching method for the silicon nitride film are plasma etching using plasma generated from a compound gas (see, for example, Patent Document 1) and wet etching using phosphoric acid, e.g., hot phosphoric acid (see, for example, Patent Document 2, paragraph [0042]).
In the plasma etching, however, it is difficult to achieve selectivity for the silicon nitride film against the thermal oxide film, so that damage on the thermal oxide film may be readily caused. Furthermore, since the plasma is used in the plasma etching, damage on the processing target object may be readily caused during the plasma etching.
On the other hand, the wet etching enables etching the silicon nitride film without inflicting damage on the target object because plasma is not used in this method. Furthermore, as compared to the plasma etching, the wet etching has an advantage in that the selectivity for the silicon nitride film against the thermal oxide film can be more readily achieved. In this method, however, management of an etching solution including management of liquid drain may be difficult.
As a solution, disclosed in Patent document 3 is a method for removing the silicon nitride film without causing damage on the thermal oxide film by way of dry-etching the silicon nitride film while using only a HF gas.
Patent Document 1: Japanese Patent Laid-open Publication No. 2003-264183
Patent Document 2: Japanese Patent Laid-open Publication No. 2006-156700
Patent Document 3: Japanese Patent Laid-open Publication No. 2008-187105